With the use of any wireless network, it depends partially on the security of the AP (Access Point) and the end user. For BBM and other types of sms, I would not worry too much about using a WiFi network (As long as it is one that you trust - Work, friend, home, etc...)

Security seems to be an interest to you, so perhaps it would be best to secure your own Mobile Wireless AP (MiFi? Not sure where you live, but most NorAm carriers have mobile wireless access available for a nominal fee) Then you would not have to worry about snoops and such.

BlackBerry has the value added bonus of being secure in and of itself.

In the US VZW requires a separate data plan for a Mifi. Unless the prices have changed recently, it's $50/month for 5 GB on a 4G Mifi. Not cheap. Although they have new shared data plans that may be cheaper.